I've read (here and here and elsewhere) that I can wire a Lamp Debuzzing Coil (LDC) in series with the load (on the hot wire between the lamp and the dimmer) in order to smooth the onset of current during the switching (i.e.: waveform mutilation) and reduce the hum.

A debuzzing coil will likely buzz as well, so the problem might not be solved just relocated. The manufacturer is aware the bulbs may buzz with this device, the troubleshooting section of the Owners Manual says to try rough service bulbs (which you've done) or switch to a non dimmer switch.
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Tester101♦Apr 13 '12 at 12:08

I see that debuzzing coils work, but two things to remember. 1) The dimmer is maxed out at 300 watts and 2) The longer the filament when dimmed it will vibrate more. While the 300w filament is bigger, it is stretched more than regular lamps.
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lqlarryApr 14 '12 at 2:24

1 Answer
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Debuzzing coils seem to start at 11 millihenries (See this example) -- which is only about 4 ohms of inductive reactance at 60 hertz.

For 300W bulbs, you want an inductor that can handle at least 3 amps, the higher the inductance the better. Beware, also, of insulation quality and heat tolerance.

I show that Digikey carries some possible candidates (none in stock). The Hammond Manufacturing, 159ZG is probably your best bang for the buck. Note that it's designed as a DC choke so it might be louder and warmer than a purpose-built LDC, like this one at Amazon. Each 300W bulb needs it's own coil.

Note that LDC's can sometimes buzz as badly as the lamp, but you can relocate LDC noise easier.

Since you mention Digikey, there is another option to stop the buzzing available to someone who knows what they are doing. You can use a rectifier bridge (Vishay GBPC3502-E4/51 or better) between the dimmer and the lamp. This works by doubling the frequency that the lamp sees and halving the amplitude swings.

A rectifier is much cheaper, makes no noise of its own, and increases incandescent bulb life.

I won't provide any more details because, if you don't already know how to install this, you could get somebody hurt. Just be sure any rectifier is mounted in a grounded metal box.

Update:
The OP tried a straight rectifier and, in this case it didn't work. But, the good news about the rectifier approach is that it's easy to change it to filtered DC. Filter the DC enough and nothing will buzz.

Use a 10uH inductor, anything more greatly increases the price without affecting performance much.

For the capacitor, you need about 200µF to start getting good filtering with 60Hz and a 300 watt load.

Here's the effects on the voltage, to the lamp, for three capacitor values. Note that the inductor is still required to deal with all the spikes, and harmonics, a dimmer induces...

We can see that by 600µF, the power is smooth enough that buzzing should be eliminated.
Here's an 820µF cap that should work nicely, cost about $4. (Just in case, monitor its temperature carefully for the first few hours of operation.)

This means I'm giving DC voltage to the bulb, right? Do I need a smoothing capacitor, or will the full-wave rectified sine waveform be enough to stop the buzzing?
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lukecycaApr 13 '12 at 15:02

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@lukecyca It is technically DC, but still has a wildly varying voltage. Which means that parts of the circuit may act similarly to AC. (e.g. Transformers may still partially work) en.wikipedia.org/wiki/Rectifier Really if you had to ask that question, I would recommend not using a rectifier.
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Brad GilbertApr 13 '12 at 17:04

I've only done this once but, yes, just the rectifier stopped the buzzing by itself.
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Brock AdamsApr 13 '12 at 21:13

I tried a GBPC3504WDI (same as your suggestion, but rated up to 400V) and the buzzing became even louder. The doubled frequency is perhaps closer to this bulb's resonant frequency. I will scrap the rectifier and might try the coil instead.
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lukecycaMay 3 '12 at 21:36

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Well that's a bummer, but don't scrap the rectifier. Add a filtering cap like you originally asked about. I guaranty that it won't buzz when it sees smother DC.
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Brock AdamsMay 3 '12 at 23:31